This invention relates to the art of device characterization for digital cameras, digital video cameras and other image pickup devices.
Device characterization, or the technique of reproducing identical images from the same scene or original image independently of the image pickup or output device employed, is recently recommended for adoption in color management areas.
Characterization is intended to achieve device-independent image reproduction or produce a uniform image from the same scene or original image independently of the type or mode of an image pickup device such as a digital camera or digital video camera or an image output device such as a display or image recording apparatus. To this end, image data are dealt with not as color/density data but as standard signals specified by the International Commission on Illumination (Commission Internationale de l'Eclairage--CIE), for example, calorimetric data in the XYZ color system.
Therefore, in order to perform characterization, color/density image data such as those of red (R), green (G), blue (B), etc. that are incorporated (imaged or read) with image pickup devices such as digital cameras and digital video cameras have to be transformed to calorimetric data in the CIE 1931 XYZ, CIE 1976 L*a*b* and other CIE standard colorimetric systems. This transformation is usually performed by employing preset color transforming conditions, specifically, preset color transforming matrices or lookup tables (LUTs). Such color transforming conditions are constructed typically by applying a known method such as the method of least squares to two kinds of data, one being device data (i.e., the data of a color chart imaged with a device of interest) and target (i.e., reference data or the actual values obtained from the same color chart by measurement with an instrument such as a spectrophotometer).
Since the output image data from digital cameras and digital video cameras differ from one imaging light source to another, the color transforming conditions such as those in a matrix or LUT form have to be constructed for each of A the imaging light source used. Stated more specifically, a color chart in the actual scene is imaged with a digital camera to obtain the device data and measured with an instrument such as a gun-type spectrophotometer to obtain the reference data, and the resulting device data and reference data are processed to set the necessary color transforming conditions. However, making the measurement of the color chart for each instance of imaging is quite cumbersome and time-consuming and, hence, is not efficient. Particularly, measurement of the reference data by using, for example, a gun-type instrument is very time-consuming.
One may think of dealing with this problem by preliminarily measuring the spectral data of the color chart with, for example, a desktop spectrophotometer, calculating the reference data from this spectral data of the color chart and the spectral data of the imaging light source and setting the necessary color transforming conditions on the basis of the calculated reference data. This method is capable of constructing the color transforming conditions by far more easily and quickly than when the reference data are measured in the actual scene. However, compared to the device data taken in the actual scene which are subject to various effects such as those caused by the position of the imaging light source and the unevenness in the quantity of light it issues, the reference data of measurement with instruments are uniform in the entire absence of such external effects. Even if the reference data obtained from the measured data are used to construct color transforming conditions, the precision in transformation is too low to achieve the correct transformation of color/density data to colorimetric data, thus failing to perform the intended characterization.